This invention is directed generally to power system control and monitoring, and more particularly to high speed, processor-based sampling of power line waveforms.
Frequency deviation of the system clock of a power metering system leads to inaccuracy in time measurement that may result in cumulative errors in time based measurements. Time based measurements include, but are not limited to, energy accumulation, real-time demand allocation and event tagging.
In one important application of the invention, power metering instrumentation for industrial applications may be used to track demand, to achieve optimum power usage versus cost patterns. Typically, power provider charges per kilowatt hour vary with demand, usually by time of day. Presently, many utilities have a dual rate structure including peak and off peak rates. With increasing sophistication, it may be possible for utilities to monitor power usage on a continuous basis, adjusting rates accordingly.
Therefore, it is important, especially for large commercial and/or industrial users to likewise monitor power consumption on an ongoing basis in order to anticipate costs, as well as provide an aid to planning for future power consumption patterns to optimize costs.
Presently, most power metering systems available to consumers are microprocessor-based and use a crystal oscillator time base. These crystal oscillators are typically very precise but exhibit long term measurable deviations from real time. Moreover, the utility-provided power grid typically exhibits some short term fluctuations or variations in the nominal 60 Hz frequency, however, this is self-correcting by the power grid.
Accordingly, the present invention proposes a comparison of the crystal oscillator time base of the consumer power metering instrumentation with the self-correcting 60 Hz frequency of the power grid. If and when variations from the xe2x80x9cgrid timexe2x80x9d are large enough, a correction can be made to the internal clock. Such corrections may be made at regular intervals, such as at the end of a given period, such as peak time and off peak time periods, on a daily basis, or otherwise to correct the xe2x80x9cend of periodxe2x80x9d measurements of time-based quantities such as kilowatt hour usage to compensate for such variations. These corrections may be relatively small on an individual basis, but may accumulate to as much as 1 to 2 percent variation in cost depending on the amount of peak and off-peak power consumption, as well as the allocation of power consumption to peak and off-peak periods, or other variable cost usage patterns or periods as defined by the utility or other provider. For large consumers, even small percentage variations can be significant on a cost basis.
This invention provides a means to reference the system clock of a metering system to the long-term frequency stability of the power system. The cumulative deviation is measured over time and then used to refine time-dependent measurements such as energy and the real-time clock used for tagging events or allocating demands, etc.
In addition, a reference system based on a Global Positioning Satellite (xe2x80x9cGPSxe2x80x9d) time source or other precision time standard can be used to monitor the same power line source. The common element between the precision time standard and the other meter(s) is power line frequency. Thus, further time corrections or correction factors may be applied to each metering system based on individual system deviations relative to the GPS or some other master standard.
In accordance with one embodiment of the invention, system clock corrections are accomplished by measuring variations of the system clock from the power line/power grid frequency. As noted above, the power line/power grid frequency provides a common time-based standard which is adjusted for long term accuracy.